Radio Frequency Identification (RFID) technology employs a radio frequency (“RF”) wireless link and ultra-small embedded computer circuitry. RFID technology allows physical objects to be identified and tracked via these wireless “tags”. It functions like a bar code that communicates to the reader automatically without requiring manual line-of-sight scanning or singulation of the objects. RFID promises to radically transform the retail, pharmaceutical, military, and transportation industries.
Several advantages of RFID technology are summarized in Table 1:
TABLE 1Identification without visual contactAble to read/writeAble to store information in tagInformation can be renewed anytimeUnique item identificationCan withstand harsh environmentReusableHigh Flexibility/Value
As shown in FIG. 1, a basic RFID system 100 includes a tag 102, a reader 104, and an optional server 106. The tag 102 includes an integrated circuit (IC) chip and an antenna. The IC chip includes a digital decoder needed to execute the computer commands the tag 102 receives from the tag reader 104. The IC chip also includes a power supply circuit to extract and regulate power from the RF reader; a detector to decode signals from the reader; a back-scattering modulator to send data back to the reader; anti-collision protocol circuits; and at least enough EEPROM memory to store its EPC code.
Communication begins with a reader 104 sending out signals to find the tag 102. When the radio wave hits the tag 102 and the tag 102 recognizes the reader's signal, the reader 104 decodes the data programmed into the tag 102. The information can then be passed to a server 106 for processing, storage, and/or propagation to another computing device. By tagging a variety of items, information about the nature and location of goods can be known instantly and automatically.
The system uses reflected or “backscattered” radio frequency (RF) waves to transmit information from the tag 102 to the reader 104. Since passive (Class-1 and Class-2) tags get all of their power from the reader signal, the tags are only powered when in the beam of the reader 104.
The Auto ID Center EPC-Compliant tag classes are set forth below:
Class-1                Identity tags (RF user programmable, maximum range ˜3 m)        
Class-2                Memory tags (8 bits to 128 Mbits programmable at maximum ˜3 m range)        Security & privacy protection        
Class-3                Battery tags (256 bits to 64 Kb)        Self-Powered Backscatter (internal clock, sensor interface support)        ˜100 meter range        
Class-4                Active tags        Active transmission (permits tag-speaks-first operating modes)        Up to 30,000 meter range        
In RFID systems where passive receivers (i.e., Class-1 tags) are able to capture enough energy from the transmitted RF to power the device, no batteries are necessary. In systems where distance prevents powering a device in this manner, an alternative power source must be used. For these “alternate” systems (also known as active or semi-passive), batteries are the most common form of power. This greatly increases read range, and the reliability of tag reads, because the tag doesn't need power from the reader. Class-3 tags only need a 10 mV signal from the reader in comparison to the 500 mV that a Class-1 tag needs to operate. This 2,500:1 reduction in power requirement permits Class-3 tags to operate out to a distance of 100 meters or more compared with a Class-1 range of only about 3 meters.
Conventional RFID tags interact strongly with the electrical and magnetic fields near them; in fact most are resonant with Q-factors ranging between 5 and 100. Unfortunately, this also means that these tags also interact very strongly with each other in ways that often prevents the tags from being read at all. The problem becomes even worse when the tagged objects are thin and flat—like poker chips, currency, documents, etc. In such a stack, the energy received by each object/tag is highly non-uniform, with the outermost objects receiving most of the energy and the interior objects shielded by the outer objects and receiving virtually no energy at all. In other words, the antenna of the outer tag serves as a Faraday shield to anything behind it. The tags on the outside of the stack will continue to work well, but the tags on the inside of the stack work very poorly if at all. For these reasons conventional wisdom was that it impossible to read a stack of tagged items.
As touched on above, in addition to blocking RF energy, the antennae interfere with RF energy in their vicinities, potentially rendering the RF signal unreadable to tags nearby. This phenomenon is best understood by considering the radar profile of the antenna. The radar profile of the antenna may often be larger than the actual physical profile of the antenna, and can be a large as 100× the physical profile of the antenna. Accordingly, the problems mentioned above can also be found in assemblies of tagged objects that are not necessarily stacked.
There are many instances where tags could be stacked or assembled in close proximity. One implementation is in poker chips. Another is paper objects such as birth certificates, paper currency, etc. Significant tag-to-tag interactions and variability also occur even with a row of tagged objects sitting on a shelf. It would be desirable to read a stack of poker chips, stack of tagged currency, file of papers, etc. in one pass via RFID technology.